This invention relates to a method for producing a fine-grained, high strength aluminum alloy material whose grain size does not unfavorably grow after the material has been subjected to a light cold working and a subsequent solution treatment.
More particularly, this present invention relates to a method for producing high strength aluminum alloy materials having a fine grain size and suitable for use in the manufacture of reinforcements for aircraft, such as stringers, stringer frames and the like.
As illustrated in FIG. 1, aircraft stringer 2 and stringer frame 3 are reinforcements which are used in the longitudinal direction and in the circumferential direction, respectively, of the inside of the aircraft fuselage 1. FIGS. 2(a), 2(b) and 2(c) are sectional views of the stringer 2 which, respectively show a cup-shaped stringer, (a) a Z-shaped sgringer and a (b) somewhat J-shaped stringer (c).
Conventionally, AA7075 alloy is well known as a typical material making for an aircraft stringer and stringer frame and has had wide-spread use in the aircraft field. Generally, the alloy is fabricated into the aircraft stringer or stringer frame by the following process.
The AA7075 alloy ingot is homogenized by heating at about 460.degree. C. to 480.degree. C. for 16 to 24 hours and hot rolled at 400.degree. C. to provide a sheet coil approximately 6 mm thick. This sheet coil is then intermediately annealed at around 420.degree. C. for 2 hours, furnace cooled and rolled to a plate of 2 to 4 mm in thickness. The cold rolled sheet coil is annealed by heating to a temperature of 420.degree. C. for 8 to 12 hours and holding the temperature for about two hours. Further, the annealed sheet coil is cooled at a cooling rate of 25.degree. C./hr to produce an O-material of the AA7075 alloy.
Further, the O-material is subjected to a stepped cold working at various cold reductions ranging from 0 to 90%, and subsequently to a solution heat treatment, thereby providing a material suitable for use in manufacturing stringers and stringer frames.
In the step of the stepped cold working, the O-material is worked to various amounts of cold reduction along the longitudinal direction, for example, as shown in FIG. 3. In FIG. 3, A shows a portion which has not been cold worked, and B, C and D show portions which have been cold worked to a relatively light reduction, a intermediate reduction and a relatively heavy reduction respectively. Such stepped cold working is practiced in order to vary the thickness according to the strength required in each portion and, as a result, to reduce the total weight of the aircraft fuselage structure.
The material which has received the stepped cold working is solution-treated and formed into the desired shape such as, for example, cup-shape shown in FIG. 2(a), by section roll-forming and the treated material is subjected to a T6 tempering treatment to provide the aircraft stringer and stringer frame.
However, the conventional stringer materials have, for example, the following disadvantages:
The O-materials used as the stringer and stringer frame materials produced from AA7075 alloy according to the above conventional method have a large grain size of 150-250 .mu.m, and if the O-materials are subjected to cold working (taper rolling) with a relatively light cold rolling reduction of approximately 10-30%, and then to the solution heat treatment, the grain size further increases. Particularly, cold reduction of 20% is known to cause the most marked grain growth. Of course, when the above conventional O-materials have received a relatively heavy cold reduction of more than 50% and then the solution heat treatment, it is possible to make the fine grain size approximately 50 .mu.m in the material. However, in practice, cold rolling reduction of a wide range of 0 to 90% is conducted on one O-material of about 10 m in length so that it is extremely difficult to achieve a grain size not exceeding 100 .mu.m over the entire length.
FIG. 4 illustrates a relationship between the reduction amount (%) by cold working and the grain size (.mu.m) of the conventional material which has been cold worked to various reductions and then solution heat treated. As can be seen in FIG. 4, in portions D, F and G which have been cold worked to a large amount of cold reduction, the grain size is small, while, in portions A, B, C and E with small cold reduction, the grain size is very large. The coarse grained portions, such as A, B, C and E, having a grain size more than 100 .mu.m, cause decrease of mechanical properties, such as elongation, fracture toughness and the like, chemical milling property, fatigue strength, etc., and further undesirable orange peel appearance and occurrence of cracks during the section roll-forming. Hence, the production of the stringers and stringer frames is not only very difficult, but also the properties of the products are not satisfactory.